On the vibrational spectra and structural parameters of methyl, silyl, and germyl azide from theoretical predictions and experimental data

High-Pressure Studies of 4-Acetamidobenzenesulfonyl Azide: Combined Raman Scattering, IR Absorption, and Synchrotron X-ray Diffraction Measurements

We have reported the high-pressure behavior of 4-acetamidobenzenesulfonyl azide (C₈H₈N₄O₃S, 4-ABSA) by in situ Raman scattering, IR absorption, and synchrotron angle-dispersive X-ray diffraction (ADXRD) measurements in diamond anvil cells with the pressure up to ∼13 GPa at room temperature. All of the fundamental vibrational modes of 4-ABSA at ambient pressure were analyzed by combination of experimental measurements and theoretical calculations using the density functional theory method. Detailed Raman and IR spectroscopic analyses reveal two phase transitions in the pressure region of 0.8-2 and 4.2 GPa, respectively, which are confirmed by the changes in the ADXRD patterns. The first phase transition in the pressure region of 0.8-2 GPa is attributed to the ring distortion and the rotation of CH₃ group, whereas the second phase transition at 4.2 GPa might be induced by the rearrangement of azide group and hydrogen bonds. The analyses of the N₃ vibrational modes suggest that the bent azide group rotates progressively upon compression, which is ascribed to the compression of the unit cell along the b axis. This study is helpful to understand the behavior of azide group and structural evolution of 4-ABSA under high pressure.

Homoleptic low-valent polyazides of group 14 elements

First examples of coordinatively unsaturated, homoleptic azido complexes of low-valent group 14 elements are reported. A simple strategy uses low-valent precursors, ionic azide transfer reagents and bulky cations to obtain salt-like compounds containing E(N3)3(-) of Ge(II)/Sn(II) which are fully characterised, including XRD. Remarkably, these compounds are kinetically stable at r.t. and isolable in sub-gram quantities.

Molecular structure of methyldifluoroisocyanato silane: a combined microwave spectral and theoretical study

The structure of methyldifluoroisocyanato silane, MeF2SiNCO (2), has been studied by molecular rotational spectroscopy. The rotational spectrum has a complicated structure from (14)N nuclear quadrupole coupling and internal rotation of the methyl group. Cavity Fourier-transform microwave spectroscopy measurements were important for providing high spectral resolution to analyze the quadrupole and internal rotation fine structure. Broadband chirped-pulse Fourier-transform microwave spectroscopy was used to achieve high measurement sensitivity making it possible to observe the lower abundance C, N, O, and Si isotopologues in natural abundance for structure determination. Analysis of the microwave spectrum of the most abundant isotopomer of MeF2SiNCO (2) yields the rotational constants: A = 3827.347(7), B = 1264.5067(14), and C = 1240.6182(11) MHz. The spectrum has been analyzed in the I(r) representation for Cs symmetry, with inclusion of the 3-fold rotor (V3 = 446(50) cm(-1)). A partial substitution structure was obtained for the C, Si, N, and O atoms. The analysis was assisted by calculations of the equilibrium structure, using a 6-311++G (3df, 3pd) basis set, with calculations at each of the B3LYP, MP2, and CCSD(T) levels. The calculated and experimental rotational constants are only consistent with a trans-orientation at each of the HCSiN, CSiNC, and SiNCO centers; there is relatively close agreement between the experimental and the theoretical structures, especially at the CCSD(T) level. In addition, the observed low value for the (14)N quadrupole coupling term (χbb - χcc) implies a wide SiNC angle, which is consistent with the calculated values: 165.3° (B3LYP), 157.6° (MP2), and 157.4° (CCSD(T)). The skeletal bending potential is discussed.

Accurate Thermochemical Properties for Energetic Materials Applications. I. Heats of Formation of Nitrogen-Containing Heterocycles and Energetic Precursor Molecules from Electronic Structure Theory

The heats of formation of 1H-imidazole, 1H-1,2,4-trizazole, 1H-tetrazole, CH3NO2, CH3N3, CH3NH2, CH2CHNO2, HClO4, and phenol, as well as cations and anions derived from some of the molecules have been calculated using ab initio molecular orbital theory. These molecules are important as models for compounds used for energetic materials synthesis. The predicted heats of formation of the heterocycle-based compounds are in excellent agreement with available experimental values and those derived from proton affinities and deprotonation enthalpies to <1 kcal/mol. The predicted value for the tetrazolium cation differs substantially from the experimental value, likely due to uncertainty in the measurement. The heats of formation of the nitro and amino molecules, as well as phenol/phenolate, also are in good agreement with the experimental values (<1.5 kcal/mol). The heat of formation of CH3N3 is predicted to be 72.8 kcal/mol at 298 K with an estimated error bar of +/-1 kcal/mol on the basis of the agreement between the calculated and experimental values for DeltaH(f)(HN3). The heat of formation at 298 K of HClO4 is -0.4 kcal/mol, in very good agreement with the experimental value, as well as a W2 literature study. An extrapolation of the CCSD(T)/aug-cc-pV(Q,5) energies was required to obtain this agreement. This result suggests that very large basis sets (> or =aug-cc-pV5Z) may be needed to fully recover the valence correlation energy contribution in compounds containing elements with high formal oxidation states at the central atom. In addition tight d functions are needed for the geometry predictions. Douglas-Kroll-Hess (DKH) scalar relativistic corrections for HClO4 and ClO4- at the MP2 level with correlation-consistent DKH basis sets were predicted to be large, likely due to the high formal oxidation state at the Cl.